Archives of the
Global Climate Change DigestA Guide to Information on Greenhouse Gases and Ozone DepletionPublished July 1988 through June 1999

FROM VOLUME 12, NUMBER 5, MAY 1999

Increased Precipitation for Central United States

In an April 19, 1999, press release, the National Center for Atmospheric
Research (NCAR) released new projections for future climate change that
were produced with the latest version of NCARs Climate System Model
(CSM). The model simulated the climate from 1870 through 2100, with two
scenarios being projected for the 21st century: a business-as-usual
projection in which atmospheric CO2 concentrations double from todays
levels to about 710 parts per million by the year 2100 and a stabilization
scenario that meets the greenhouse-gas-reduction objectives of the UN
Framework Convention on Climate Change. The projections also employed a
more realistic scenario for the future emissions of sulfur dioxide than
had been considered previously.

Initial analysis of the 21st century model results show substantial
global-scale warming during the next century in both simulations:
approximately 2° C for the business-as-usual case and 1.5°C for the
stabilization case, three to four times larger than the warming that has
occurred over the 20th century. The simulations indicate that CO2
stabilization would have a more marked influence on temperature over
Eurasia than over North America. However, no clear separation would occur
between the business-as-usual and stabilization cases until around 2060.
This multidecadal lag between the changes in emissions pathways and
changes in the modeled climate results from the large thermal inertia in
the climate system, especially the oceans.

The model results also show appreciable precipitation changes that vary
markedly from region to region and season to season as well as between the
two cases studied. In the business-as-usual case, winter precipitation
increases by more than 40% over the U.S. Southwest and central Great
Plains. The slight changes along the eastern coastal areas are generally
towards drying in winter and moistening in summer. In the stabilization
case, the changes in the Southwest and Great Plains regions are reduced
substantially.